No, an assonance is repeating of a vowel sound, which sorrow-sorrow is not repeating the vowel sound, it's just repeating the word itself. An alliteration is when a sentence or phrase repeats the first letter in each word, and that's clearly not the case. So what you have is what I have stated before, and that is a metaphor! Metaphors literary figures of speech that describe a subject by asserting that they are, on some point of comparison, the same as another otherwise unrelated objects.